As a method to lower resistivity of a p-type semiconductor which becomes to have p-type conductivity by doping impurity, conventional techniques using electron beam irradiation apparatuses are well known, for example, as shown in “Present Status and Future Prospects of GaN-based Light Emitting Devices” in Journal of Applied Physics Vol. 65, No. 7, 1996.
Such an electron beam irradiation apparatus which has been proposed in conventional art arranges samples (objects to which electron beams are irradiated, e.g., semiconductors) in vacuum, irradiates electron beams in vacuum and lowers resistivity of the semiconductor. Recently, because it consumes a lot of time to evacuate a chamber, a method for planar irradiation in which a great number of electrons are generated and electron beams are irradiated in plane in an ambient air has been proposed. That is disclosed in, for example, Japanese Patent Application No. 2000-253424 and No. 2000-254306 which are invented by the inventors of the present invention.
However, even when planar irradiation of electron beams is carried out, problems persist in deteriorating the surface of the semiconductor.
So, in order to prevent the semiconductor doped with p-type impurities from being deteriorated by electron beam irradiation, the inventors of the present invention invented a method in which a thin metal layer is formed on the surface of the semiconductor and electron beams are irradiated over the metal layer. As a result, lowering resistivity of the semiconductor is achieved without generating a damaged layer at the surface of the semiconductor (Japanese Patent Application No. 2000-254306).
The method can lower resistivity of the semiconductor, however, the metal layer after electron beam irradiation is oxidized and cannot be removed completely in consequent etching treatment. In short, when an electrode is formed on the semiconductor, contact resistance of the semiconductor increases and driving voltage cannot be decreased sufficiently. And if this method is applied to an optical device such as an LED, a laser device and a light-receiving device, optical transmittance factor of the device is decreased.